Donald Trump’s
defence secretary has warned North Korea it would face an “effective
and overwhelming” response from the US if it used nuclear weapons.

Speaking in South
Korea on Friday, James Mattis reassured the government in Seoul that
the US would retaliate should its northern neighbour launch any
attack.

Mattis, the first
senior figure in the new US administration to make an overseas visit,
is expected to offer similar security reassurances to Japan despite a
suggestion by Trump during the election campaign that Washington’s
commitment to its closest ally in the region could weaken unless
Tokyo paid more towards the cost of hosting American troops.

“Any attack on the
United States, or our allies, will be defeated, and any use of
nuclear weapons would be met with a response that would be effective
and overwhelming,” Mattis said in Seoul before leaving for Japan,
where he will meet the prime minister, Shinzo Abe, and the country’s
foreign and defence ministers.

Abe, who claimed to
have established a rapport with Trump during their first meeting in
New York last November, will be buoyed by reports that Mattis does
not intend to broach the subject of host-nation payments towards
supporting almost 50,000 US troops based in Japan.

The US also has
28,500 troops in South Korea, mostly ranged along the heavily armed
border separating it from the North.

Japan has said that
it pays its fair share towards maintaining a US troops presence,
mainly on the southern island of Okinawa.

Abe, who is to meet
Trump in Washington next week, told MPs he would remind Mattis about
“the significance of the Japan-US alliance”.

Mattis’s remarks
in Seoul come amid concern that North Korea could be preparing to
test a new ballistic missile, in what could be an early challenge for
Trump’s administration.

North Korea, which
regularly threatens to destroy South Korea and its main ally, the
United States, conducted two nuclear tests in 2016 and more than 20
missile tests, in defiance of UN resolutions and sanctions.

In his New Year’s
speech the North Korean leader, Kim Jong-un, said Pyongyang was in
the “final stages” of developing an intercontinental ballistic
missile (ICBM). Trump responded with a tweet that said “It won’t
happen!” but has not explained how he would prevent a North Korean
ICBM test launch.

The North appears to
have also restarted operation of a reactor at its main Yongbyon
nuclear facility that produces plutonium suitable for its nuclear
weapons programme, according to US thinktank 38 North.

North Korea’s
actions have prompted the US and South Korea to respond by bolstering
defences, including the deployment of a US missile defence system,
known as terminal high-altitude area defence (Thaad) in South Korea
later this year.

Mattis and his South
Korean counterpart, Han Min-koo, on Friday agreed to deploy Thaad “as
a defence system solely against North Korea’s missile threat”,
the South Korean defence ministry said in a statement

China has objected
to Thaad, saying it will destabilise the regional security balance,
and some South Korean opposition leaders have called for it to be
delayed or cancelled.

Han said the
agreement on Thaad indicated that South Korea would continue to enjoy
strong US support under Trump. “Faced with a current severe
security situation, secretary Mattis’s visit to Korea … also
communicates the strongest warning to North Korea,” he said.

Once fully developed
a North Korean ICBM could threaten the United States, which is about
9,000km away (5,500 miles). ICBMs have a minimum range of about
5,500km (3,400 miles) but some can travel 10,000km (6,200 miles) or
more.

Former US officials
and other experts have said the United States essentially has two
options when it comes to trying to curb North Korea’s
fast-expanding nuclear and missile programmes: negotiate or take
military action.

Some analysts say
China’s opposition to Thaad makes it less likely that Beijing will
act to rein in North Korea – a demand made by Trump and Barack
Obama.

“Deepening
tensions between China and the US adds to the North’s strategic
value in the eyes of China,” said Lee Ji-yong, a professor at the
Institute for Foreign Affairs and Security. “It will make it more
difficult for the US to persuade China to cooperate in pressuring the
North to give up its nuclear arsenal.”